1. Field of the Invention PA1 1. The CM process will form a beam on the largest CM signal present, nulling every other signal. PA1 2. Having previously formed a beam on a signal, the CM process will not form a beam on another signal until the first signal's power falls near or below the noise power of the system.
This invention relates to that disclosed and claimed in co-pending U.S. Patent Application, Attorney Docket TI-24746, filed on the same date as this application. This invention relates to the use of adaptive antennas in communications systems, and in the preferred embodiment, to the use of least squares estimation to control the adaptive antenna by determining the amplitude of supervisory tones in an AMPS system.
2. Background of the Invention
The Analog Mobile Phone System, or AMPS, is the standard cellular phone system in the United States. As cellular phone usage grows, other competing systems are becoming prevalent, but the AMPS system is forecast to be the principal US cellular network for the near future. While the invention has more general applications, the preferred embodiment will be disclosed in the context of the AMPS system.
In AMPS, each user is given two 30 kHz regions of the spectrum, one for transmitting to the base station (the uplink) and one for receiving from the base station (the downlink). Each 30 kHz spectrum carries one direction of the two way communication. The AMPS signal is a carrier signal, frequency modulated (FM) by the analog data to be communicated. Supervisory audio tones or SATs are also frequency modulated onto the carrier. The SATs are tones at 5,970, 6,000, or 6,030 Hz transmitted by the base station downlink (each base station uses one of the tones). The SATs or identifier components are used to aid in identification of the desired signal in a background of interfering signals which may be present in the received data. The mobile or remote station determines which tone is being used by the base station and regenerates that tone on the uplink. The presence or absence of the SAT is used in the call setup protocols for initialization of the communication and also in the handoff protocols when a mobile station travels from the region of one base station to another.
FIG. 1 illustrates a prior art apparatus which can be used to determine which of the possible SAT tones has been received with a given FM signal. The FM input signal 2 is demodulated in demodulator 4 and is then coupled to the inputs of three parallel bandpass filters 6, 8, and 10, each centered at one of the SAT frequencies, and each having a bandwidth of 30 Hz. The propagation time of the signal through the filter is roughly the inverse of the bandwidth, that is about 33 msec in the present case. Because of the presence of surrounding cells, the received FM input signal may contain energy at any or all of the SAT frequencies. The filter outputs represent the amount of such energy at each of the three frequencies. The amplitudes of SAT signals at each of the SAT frequencies are then estimated by a short averaging process in amplitude estimators 12, 16, and 18. SAT amplitude comparison logic circuit 20 then determines whether or not the signal having the largest SAT signal is at the correct frequency. While the 33 msec filter delay time is acceptable in some applications, as will be more fully explained, that time is unacceptably long in others.
AMPS uses 832 30 kHz full-duplex channels. Each two-way communication channel is composed of two frequencies. The 832 frequencies allocated for the uplink are in the band from 824 to 849 mHz while the downlink frequencies are in the band from 869 to 894 mHz. To encourage competition, the FCC has mandated that there be the possibility of two major service providers in each metropolitan area. Thus, each service provider is given 416 of the 832 channels. Of these, 21 channels are used as control channels for controlling access and handoff, leaving 395 channels for phone calls.
The network is built on the concept of a cell, which is the region serviced by a base station. A metropolitan network is commonly composed of many tens of these cells. Each cell uses a subset of the available channels, typically about 1/7 of the 416 available. Thus, the frequencies assigned to a given base station are reused at base stations elsewhere in the network. The base stations using the same frequency assignments must be separated by some distance in order to avoid interference caused by multiple users on the same frequencies. In other words, the current concept uses the signal attenuation associated with distance to control interference in the network.
As the use of such systems grows, many of the metropolitan cell sites operate at or near capacity. Because of the need for spatial attenuation between base stations using the same frequencies, it is difficult to add channels (frequencies) to the capacity limited base stations without causing unacceptable interference to other base stations.